@Ibowl:

You need to give your specs; what conditions you generally are playing on, and the speed(to the best of your knowledge) that you throw.

I am also assuming that moving in, and playing a new oil line has already been tried?

I have not thrown any Motiv pieces, though the primal Rage seems to be a popular choice; versatile and a lot of different bowling styles have been able to use it.

In the Roto line, you might try the Uproar, to give you added length with a small flip at the end of the pattern. Roto Grip really offers you a wide array of choices to choose from. A Deranged drilled the same as your Assassin, with a slightly scuffed cover might give you the added length and control you seek, also, or An Asylum drilled a little weaker than your Assassin would give you the option of a stronger piece, but a smoother reaction for controlling the transition as well.

But it all depends on what you like to see from a ball. I would go to bowlingballs.com and watch some of the vids on there to kind of gauge what you'd like to see performance-wise.

Personally, I like to have a stronger pearl to go to when my control ball begins to hit weak (on a THS).  If you are the same, a Primal Rage would be the Motiv option.  If you like to have a weaker pearl to transition to, then the Tribal would be the better option from Motiv.

I do not know Roto Grips line up well enough to recommend.